This invention relates generally to human hearing, and more specifically to the design and surgical insertion and positioning of a totally implantable cochlear prosthesis.
Human deafness results from numerous sources including trauma, ear infections, congenital factors, ototoxic effects of some antibiotics, and from diseases such as meningitis. Sensorineural damage (damage to the hair cells in the cochlea) is the largest single form of hearing loss. In a healthy ear these hair cells convert acoustic signals in the inner ear to electrical signals that can be interpreted by the brain as sound. It is estimated that over 7% of the U.S. population is affected by sensorineural deafness, and one in a thousand infants is born totally deaf. Extrapolating these percentage figures, it is estimated that there are more than 30 million people in the world who are profoundly deaf.
Currently available cochlear prostheses generally use an internal part containing an electrode array for insertion into the cochlea, and at least some electronics in a sealed subcutaneous package. Information is inductively transmitted through the skin via a head-mounted (magnetically held) device to the subcutaneous package, which package is designed to receive and process coded information for further transmission to the electrode array. The battery and a major part of the acoustic signal-processing electronics are housed in an externally worn device. These designs are not aesthetically pleasing, nor practical for sleeping or for other activities such as diving, swimming or other physical activities. Besides the inconvenience of said designs, there is a social stigma, especially for children, in wearing a head-mounted device, connected to an externally visible apparatus. Additionally, the length of surgery and the surgical complexity of implanting current commercial cochlear prostheses is significant, especially for very young and for very old persons. Said implant procedure involves exposure of the mastoid cortex and external auditory canal of the implanted ear via elevation of a postauricular skin flap, generally requiring 2-4 hours where the patient is totally anesthetized, with the inherent medical risks of total anesthetic. Finally, the cost of said cochlear prostheses is high, limiting the availability of this technology mostly to the wealthy industrialized countries.
In spite of the surgical risks, complexity and device costs, currently available cochlear prostheses do provide a major improvement over the alternativexe2x80x94total deafness. However, there are still great differences in hearing percepts amongst implanted patients. Some patients after implantation are able to use the telephone, while others can only perceive environmental sounds.
Such cochlear prostheses are commercially available from a number of companies worldwide, for example, from Cochlear Limited, Sydney, Australia; Advanced Bionics Corporation, Sylmar, Calif., U.S.A.; Med-El Medical Electronics, Innsbruck, Austria; PHILIPS-Antwerp Bionic Systems N.V./S.A., Edegem, Belgium; and MXM Medical Technologies, Vallauris, France. A comprehensive introduction to the development of cochlear implants is given in, for example, xe2x80x9cCochlear Prostheses,xe2x80x9d edited by G. M. Clark, Y. C. Tong and J. F. Patrick, distributed in the U.S.A. by Churchill Livingstone Inc., New York, N.Y. (ISBN 0-443-03582-2), and in xe2x80x9cThe Cochlear Implant,xe2x80x9d (ISSN 0030-6665) by T. J. Balkany, editor of The Otolaryngologic Clinics of North America, Vol. 19, No. 2, May, 1986. Some of the early cochlear implant work is also described in, for example, U.S. Pat. 4,357,497; 4,419,995 and 4,532,930.
Although a commercial totally implanted cochlear prosthesis does not yet exist, A. J. Maniglia in U.S. Pat. 5,906,635 and A. J. Maniglia, et. al. in xe2x80x9cThe Middle Ear Bioelectronic Microphone for a Totally Implantable Cochlear Hearing Device for Profound and Total Hearing Lossxe2x80x9d, in The American J. of Otology, 20:602-611, 1999, describe a possible design for a totally implanted device. Their proposed device uses a permanent magnet attached to the malleus which movement is detected by a nearby coil so as to detect vibrations of the ear drum. Said design uses a relatively complex arrangement of hermetically sealed packages, rods and brackets. A separate external RF linked unit is used to control the volume and on/off functions. The Maniglia prior art has a number of shortcomings namely: (a) said design is not MRI compatible, (b) a large excavation is required in the mastoid cavity to anchor and position said device, (c) mounting a magnet directly onto the ossicular chain can reduce the blood supply, creating the danger of future necrosis of the bone, (d) the device is not suitable for use in newborns, (e) there is no manual panic safety off switch, and (f) there is no allowance for future head growth between the various permanently anchored mounting brackets.
A totally implantable cochlear prosthesis of the present invention is designed to address the limitations of the prior art and to: (a) significantly improve the aesthetics and practical problems of coping with the relatively bulky external components of conventional cochlear prostheses, (b) allow the user to hear more naturally via the auditory canal (ear canal) and (c) enable versatile speech processing algorithms to be used to stimulate the auditory nerve cells for improved speech perception. Additionally, the implant is designed to reduce the surgical complexity and time needed by the surgeon for device implantation.
In one of its aspects, the invention comprises two parts, namely:
1. An implanted part, and
2. An external part
The implanted part itself embodies further aspects of the invention. The implanted part comprises four principal components:
(a) a housing
(b) a coil
(c) a microphone, and
(d) an electrode array
The implanted part is a stand-alone fully functional cochlear prosthesis, such functionality only limited by the capacity of the battery, or other storage device, contained within the implant housing. Said implanted part is comprised of a housing, coil, microphone and electrode array, where all components in contact with body tissues are biocompatible, and all non-biocompatible materials are substantially hermetically sealed with biocompatible materials. Such hermetic sealing is required to prevent ingress of biological fluids into the enclosure, creating device malfunction, and/or to prevent leakage of non-compatible material into the surrounding biological tissues. Medical devices such as conventional cochlear prostheses or pacemakers commonly use titanium enclosures (or ceramic enclosures with metal sputtered bonding surfaces) which are hermetically sealed using laser welding. For a brief review of sealing concepts, see for example, S. J. Rebscher""s article in Chapter 4 (Cochlear Implant Design and Construction), published in Cochlear Implants, ed. Roger F. Gray, Croon Helm Ltd., London, 1985.
The preferred embodiment is to position the housing and coil substantially against the skull, posterior to the pinna, with the electrode array entering the middle ear via a mastoid cavity and facial recess approach, using either a modified version of a cochleostomy in conjunction with a transcanal approach. Such surgical cavity also allows for convenient positioning of the microphone underneath the skin in the posterior wall of the external auditory canal.
The connections from the microphone and electrode array to the housing are comprised of, preferably, platinum and/or gold wires formed lithographically, said wires encapsulated in a bioinert polymer, such as the fluorocarbon polymer FEP. Said connections are xe2x80x9ccorrugatedxe2x80x9d to enable the connections to expand with minimum stress during surgical handling, skull growth, and any post surgical movement of the implanted components.
(a) The Housing
The housing, which contains the wiring interconnections, electronics, battery, means for setting volume and safety xe2x80x9coff/onxe2x80x9d, and, in an alternate embodiment, magnets, is positioned substantially against the skull behind the outer ear (or pinna) where said housing can be conveniently sutured to the overlying tissue or attached to the underlying bone with sutures, or titanium screws. Said housing comprises two sections, each section having a relatively thin and rounded profile at the edges, where such sections are joined by a pliable (or bendable) bridge to allow the surgeon to bend said bridge so as to better fit the two housing sections to the curved surface of the skull.
The key components within the housing sections are mounted on an insulated substrate (containing hermetically sealed electrical lead-throughs), where the substrate is comprised of ceramic or glass or a combination thereof, but preferentially ceramic, where the substrate is further bonded to an underlying cable, preferably a ribbon type cable, containing wires, where said ribbon cable is preferably comprised of a bioinert polymer film encapsulating lithographically formed wires. Openings within the polymer film allow electrical connections to be made from said conducting wires to the overlying ceramic substrate lead-throughs. The polymer film is then further bonded to an underlying gold foil substrate. Since some of said components may be comprised of non-biocompatible materials, they require complete hermetic or hermetic like sealing on all surfaces. The preferred embodiment to achieve such all surface sealing is to use a medical grade epoxy to cover said components (mounted onto the ceramic substrate), where said epoxy also provides shape to the overall housing. Alternately, any conformable potting material, such as a bioinert polymer, can be used as an encapsulant material to cover said components. Since epoxy, or other, encapsulation (over the ceramic substrate) does not provide a true hermetic or hermetic like seal, said encapsulant surface is coated first, preferably, with a thin coating of vacuum deposited gold, or alternately, with an electroless gold deposition, where such a first coating of gold is subsequently thickened by an electro deposition of gold. Alternate embodiments include coating the electro deposited gold layer with titanium (or platinum), and/or with medical grade silicone. The same gold coating processes are also used to provide a gold seal between the bridge connecting the two housing sections, and the point at which the connectors from the microphone and electrode array enter the housing bridge.
Since the encapsulant substantially covers the components within the two housing sections, in one embodiment, hollow beads or microcontainers containing, for example, helium gas and or other leak detection gases, are positioned within the encapsulant to act as a leak detection gas using standard quadrupole mass spectrometry, such gas leak detection system acting to verify the extent of the housing hermeticity.
In further embodiments, a laser, ultrasonic or electric welded titanium, ceramic (or a hybrid of both) enclosure may be used as the housing. For any ceramics used, these would need to be coated with titanium at any locations required for subsequent welding.
The various electronics within the housing should draw minimal power, and occupy a relatively small volume. The preferred embodiment is to use a custom designed low power hybrid analog-digital ASIC microchip for processing signals from the microphone and for controlling the current pulses to the cochlear electrodes. Alternately, a more conventional DSP based electronics design can be used, such design using a combination of standard and/or custom circuits. However, such a DSP design generally requires more power and space. The key advantages of using an analog-digital microchip are that such a device has very low power requirements, occupies a very small volume, and is inherently more reliable than using numerous discrete electronic components.
For the preferred embodiment, a thin film secondary (rechargeable) lithium battery is located in one housing section. Preferably, such a battery should have high charge density, be substantially rechargeable over tens of thousands of charge/discharge cycles, and use a safe solid state electrolyte. Alternately, a rechargeable nickel metal hydride battery can be used, although such a battery has a relatively low charge/discharge cycle capability, thus requiring surgical replacement every few years. A further alternate embodiment is to use a simple capacitor for short term storage of electrical power, in lieu of a battery storage system, where such a design would be used in conjunction with an externally-worn head mounted inductively coupled power input device worn continuously by the implantee.
Control of power and volume to the implanted part can be accomplished in a myriad of conventional ways, such as an RF link, IR light pulses, or acoustic signals through the skin. However, the invention uses a mechanical method to operate a panic xe2x80x9coff/onxe2x80x9d control, and an overall xe2x80x9cvolumexe2x80x9d control. A panic shut off of the electronics is preferred in the event of device failure which could send excessive current to the cochlea electrodes, damaging the neural processes therein. However, such a safety shut off is preferably independent of the control electronics and should be capable of being quickly implemented by the user, in any setting. Ideally, the panic xe2x80x9coff/onxe2x80x9d control does not need a sophisticated external device to activate said control. According to the invention, a panic xe2x80x9coff/onxe2x80x9d control incorporates a pressure switch (located within a housing section), preferably a piezoceramic or piezocrystal disc (or a bimorph) mounted on a thin metal diaphragm, where such diaphragm can be slightly bent by the user by simply pushing (ie. with a finger) against the skin behind the pinna, thus producing an electrical signal from the piezoceramic that can be used by the electronics to control the panic xe2x80x9coff/onxe2x80x9d switch. In an alternate embodiment, a piezoelectric film such as PVDF (polyvinylidene fluoride), or copolymers of PVDF, produced by Measurement Specialties, Inc. of Valley Forge, Pa. can be used, where said piezo film is held in a xe2x80x9cdrum-likexe2x80x9d configuration. The surface of said film can be (indirectly) pressed, through the skin surface, by the user by simply pushing (ie. with a finger) against the skin behind the pinna. Such pressure slightly deflects the PVDF film membrane inducing sufficient current and voltage to the control electronics to disengage power to the electrodes in the cochlea. In an alternate embodiment a mechanically-actuated electrical switch for the panic xe2x80x9coff/onxe2x80x9d switch can also be used in place of a piezo type device. To assist the implantee with a tactile sense as to when a contact is made, a snap dome can be used in conjunction with the piezo type panic xe2x80x9coff/onxe2x80x9d switches, or the mechanically-actuated electrical switches. Also, the use of a snap dome provides for a better signal since the operation of the piezo type or mechanical type switch will be suddenly activated when the pressure on the snap dome reaches a certain value.
In yet a further embodiment, a second panic xe2x80x9coff/onxe2x80x9d control using a magnetic reed switch is located in a housing section, which switch is normally in the closed mode, except when exposed to a magnetic field, such field opening the reed switch, disengaging power to the electrode array. The user can simply hold a magnet against the skin substantially over said switch thereby inducing a magnetic field through the skin to activate the reed switch. Removing the external magnet would close the reed switch thereby activating the implanted prosthesis again.
A piezoceramic or piezocrystal disc (or bimorph) or, alternately, a PVDF type piezo film can also be used as a xe2x80x9cvolumexe2x80x9d control switch, similar to the panic xe2x80x9coff/onxe2x80x9d control design, to enable the user to adjust the volume xe2x80x9cupxe2x80x9d or volume xe2x80x9cdownxe2x80x9d so as to achieve the preferred sound level. Said xe2x80x9cvolumexe2x80x9d control can be mounted within one of the housing sections, and activated through the skin surface by the user pushing (ie. with a finger) against the skin behind the pinna, such pressure producing sufficient current and voltage from the piezoceramic, piezocrystal or piezo film material to activate the appropriate microelectronic circuits. In an alternate embodiment a mechanically-actuated electrical switch can also be used in place of a piezo type device. To assist the implantee with a tactile sense as to when a contact is made, a snap dome can be used in conjunction with the piezo type panic xe2x80x9coff/onxe2x80x9d switches, or the mechanically-actuated electrical switches. Also, the use of a snap dome provides for a better signal since the operation of the piezo type or mechanical type switch will be suddenly activated when the pressure on the snap dome reaches a certain value.
(b) The Coil
An external coil and an implanted coil are used to inductively couple electrical power and data through the skin surface. This technology is well established: see for example, Chapter 7 of xe2x80x9cDesign and Fabrication of an Implantable Multichannel Neural Stimulatorxe2x80x9d, by M. Soma, June 1980, Technical Report No. G908-1, National Institute of Health Contract No. N01-NS-5-2306; and xe2x80x9cHigh-Efficiency Coupling-Insensitive Transcutaneous Power and Data Transmission Via an Inductive Linkxe2x80x9d, C. M. Zierhofer, et. al., IEEE Transactions on Biomedical Engineering, vol. 37, no. 7, July 1990. Also, a myriad of patents addressing various methods for improving or modifying such inductive links have issued, see for example, U.S. Pat. Nos. 4,441,210; 5,070,535; 5,279,292; 5,876,425; and 5,891,183. Unfortunately, the inductively coupled signal is attenuated by metallic layers present over the coil or metallic parts (such as magnets) contained within the coil perimeter. Such attenuation is also a function of the frequency of the transmitted signal.
The preferred embodiment of the present invention uses two RF coupling coils which are not RF shielded, and locates the implanted coil outside the housing. The coil comprises a number of turns of a biocompatible metal, such as platinum, titanium, gold, tantalum, iridium, rhodium, or rhenium, or any combination thereof, encapsulated in a bioinert polymer carrier, preferably a polyfluorocarbon such as FEP. The coil can be fabricated using discrete wires, or preferably, using lithographed tracks. An added advantage of such a design is that the coil has a very thin profile, and is flexible, thus fitting easily to the curved skull surface. Also, the center area within the coil may be made substantially xe2x80x9copenxe2x80x9d, such a design minimizing the surface area separating the skull and overlying skin. A further advantage of the invention is the absence of a subcutaneous magnet or magnetizable material, within the coil perimeter, or housing, thus enabling an implantee to have an MRI scan without explantation of part or all of the implanted prosthesis. Commercial cochlear prostheses containing implanted magnets can restrict the user from having an MRI scan (see for example, B. P. Weber, et. al., vol. 19, no. 5, pages 584-590, 1998, American J. of Otology).
Also, it is advantageous to include a method for identifying the model, date or serial number of the implant, post surgery, using a non-invasive technique such as X-ray, CT scan or other medical imaging methods. Such identification can be conveniently accomplished by locating numbers, letters, marks or other identifying symbols substantially on the inside or outside perimeter of the coil, where such identifying markings can be conveniently fabricated lithographically in, for example, platinum or gold within a polymer film, such film also containing the coil wire, or lithographed coil tracks. Alternately, said markings can also be located anywhere on the implanted part where such markings would be identifiable using medical imaging methods.
(c) The Microphone
In the preferred embodiment a small microphone is encapsulated in a bioinert material such as titanium, gold, platinum, iridium, tantalum, rhodium or rhenium, or a polymer, such as a polyfluorocarbon, or any combination thereof, but preferably a titanium housing, said encapsulated microphone anchored in the posterior wall of the external auditory canal. A small mastoidectomy cavity is created surgically and the skin of the posterior wall of the external auditory canal is elevated. The bony wall between the mastoid cavity and the external auditory canal is thinned down to match the dimensions of the encapsulated microphone. A hole is created in the posterior wall of the external auditory canal about half way between the tympanic ring and the meatus of the external canal. This hole is also made substantially to the dimensions of the encapsulated microphone, using an appropriately sized drill bit and a custom designed hand tool. The thin diaphragm, preferably titanium, covering the sound input part of the encapsulated microphone, is fitted so as to lie underneath the skin of the posterior wall of the external auditory canal. In an alternate embodiment, a protective cap is placed over the titanium diaphragm during handling to protect it from damage.
Sound entering the user""s auditory canal, will be received by the microphone, with the acoustic signal converted to an electrical signal within the microphone, where the electrical signal is then sent to the electronics within the housing for signal processing. One or more of numerous signal processing strategies may then be used to send electrical current pulses to the electrode array for stimulating the auditory nerve fibers, such stimulation being perceived by the user as sound.
(d) Electrode Array
The description of the preferred embodiment of the electrode array is disclosed in U.S. patent application Ser. No. 09/376,918 which is incorporated herein by reference. However, any compatible electrode array could be attached to the housing.
An external part is required from time to time to recharge the battery, or other storage device, contained within the implanted housing. Such recharging of the internal battery (or other storage device) can be conveniently accomplished by, for example, using an external part to inductively couple power through the skin surface to an implanted receiving coil contained within, or preferably outside of, the implanted housing whereby the electrical power received by the implanted coil is used to charge the implanted battery (or storage device). Such external part is preferably held to the head either by mechanical means or, alternately, by magnetic means. For the preferred embodiment, the external coil is mounted behind the pinna on the arm of the frame of eye glasses, such that the external coil substantially overlays the implanted coil. Power to the external coil can be provided by including a battery (which itself may be rechargeable) and control electronics attached to the eye glass frame, or via a wire to a body-mounted power source. Alternately, the external coil can be mounted behind the pinna using a standard xe2x80x9cBTExe2x80x9d device (like a behind-the-ear hearing aid package) to anchor and position the external coil substantially over the implanted coil. Power to the external coil can be provided by mounting the battery and control electronics directly on the BTE device, or via a wire to a body mounted power source.
A further embodiment is to use an arrangement of external and implanted magnetic fields to both hold the external part against the head and to also rotationally align said external device, such rotational alignment enabling the external coil and implanted coils to be substantially coaxial to achieve optimum inductive coupling. Such rotational alignment between the external and internal magnetic fields is necessary since the magnets and coil in each of the external and implanted parts are separated (ie. the magnet is not placed within the coil perimeter).
A yet further embodiment is to use a conventional method of alignment whereby opposing external and implanted magnets (which are each coaxial with and contained within the coil perimeter) are used such that the external part will be magnetically held against the head to position the external coil substantially over the implanted coil.
The external and implanted coils can also be used to transmit and or receive coded information to/from the electronics in the implanted housing to control or monitor electrical parameters in said housing.
In a yet later embodiment, the external part can be connected to a computer such that data can be generated by said computer to transmit electronic signals to the implanted part to change the parameters within the electronics of the implanted part.
For the embodiments where the external part contains a battery, such battery may be a secondary (or rechargeable) battery, or, in an alternate embodiment, a primary battery that is discarded after usage.